I had an MRI done and he said my labral and rotator cuff were fine, actually good.

I find it really hard to activate my rear delts when trying to work them ( yes i do LOW weight ) but i just cant seem to stimulate them and get a good burn? could this be why i've got that pain?

Did he have a look at your posterior capsule? You can get tears in those too. I find the best way to activate rear deltoids is to bend over with light weights, and reverse fly then with pinky fingre slightly higher, then pausing at peak contraction for a few seconds, then relax and repeat